Add form pre alter hook

This request is similar to:

1. #1899946: Add Reference Form Alter Hook
2. #1899916: Move Hook Entity Form Alter

I suggest that we need a 'pre' version of the inline_entity_form alter hook, to allow custom modules to alter the entity & form_state before the normal entity form is built.

My use case is that I'm trying to pre-fill one of the field values on the embedded entity form and the only way that this can be done is to 'get in' there before the field_attach is run.